Method and apparatus for acoustically driven media filtration

ABSTRACT

A method and apparatus for acoustically enhanced particle separation uses a chamber through which flows a fluid containing particles to be separated. A porous medium is disposed within the chamber. A transducer mounted on one wall of the chamber is powered to impose on the porous medium an acoustic field that is resonant to the chamber when filled with the fluid. Under the influence of the resonant acoustic field, the porous medium is able to trap particles substantially smaller than the average pore size of the medium. When the acoustic field is deactivated, the flowing fluid flushes the trapped particles from the porous medium and regenerates the medium. A collection circuit for harvesting the particles flushed from the porous medium is disclosed. Aluminum mesh, polyester foam, and unconsolidated glass beads are disclosed as porous media.

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/019,770, filed Jun. 14, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for separatingparticles from a fluid suspension by filtration. In particular thepresent invention relates to an acoustically driven method and apparatusfor filtering fine particles in a filtering medium having an averagepore size substantially larger than the diameters of the particles beingfiltered. As used herein, the term “particles” encompasses solids,immiscible liquid droplets, gas bubbles, and other types of discretematter that might be suspended or entrained in a fluid.

Filtration of fluid suspensions containing submillimeter-size particlesis of fundamental importance in many chemical and biological processingapplications. Conventional separation approaches include physicalscreening techniques (mechanical sieves, beds of filtration media, orporous membranes in which the fluid passes through pores smaller thanthe size of the solid particles being collected), gravity-driven methodsthat accomplish separation based on the difference in densities of theparticles and the host fluid (centrifugal and settling techniques), andprocedures that involve external fields (such as electrical or magnetic)to enhance the quality or rate of separation based on specific systemproperties.

Filtration of fine solid particles is often very difficult, however, dueto the strong interactions between the solids and their host liquid. Inthe case of conventional screening methods, high pressure drops or slowprocessing rates often result from the plugging of membranes or theblocking of pores by the particles. Furthermore, back-flushing of amembrane to regenerate the filtering medium often is difficult due tostrong interactions between the particles and the filter substrateitself Moreover, suspensions of immiscible liquid droplets and gasbubbles typically cannot be filtered, as these types of particles arecapable of distortion or splitting to pass through the pores in thefiltering medium.

In the past few decades, methods based on the use of ultrasonic standingwave fields have been developed for separation of particles from liquidstreams without reliance on filtration media. These methods exploit thedensity and/or compressibility difference between suspended particlesand the host liquid to yield sharp, highly efficient separation ofparticles using resonant acoustic fields In the case where aone-dimensional sound field is used, the particles are organized intothin parallel bands separated by a one-half wavelength spacing. Theparticles then are separated from their host fluid by placing closelyspaced physical barriers between the bands of particles, transportingparticles in the opposite direction of the flowing host liquid by usingpseudo-standing waves, or relying on gravity to settle the swarms ofparticles organized by the acoustic field. These particle-harvestingtechniques, however, can be problematic for practical applications. Thefirst two approaches are difficult to achieve mechanically because ofthe typical small separation distances involved, and the third techniquecan be hindered by slow sedimentation rates.

Acoustic fields also have been applied in membrane or sieve filtrationprocesses. In these applications, however, intense ultrasonic fields areused to create vibrations in the filtering medium (or in the cake formedabove the medium) for the limited purpose of preventing or reducingclogging. Essentially, these ultrasonic fields are applied to freeparticles from the filtering medium, not to enhance the medium'sfiltering efficiency.

The concept of combining a porous filtering medium with an externalfield to enhance particle separation has proven useful indielectrophoresis and high-gradient magnetic separation. These methodsare limited, however, to particles having certain electrical or magneticproperties and therefore are not suitable for a wide variety ofapplications.

The present invention is intended to utilize the imposition of anacoustic field to increase the efficiency of a porous filtering medium.

In particular, the present invention is intended to enable a porousmedium to collect particles up to three orders of magnitude smaller thanits average pore size.

The present invention also is intended to provide an acoustically drivenenhanced filtration system that does not rely upon the electrical ormagnetic properties of the particles being separated and is capable ofsuccessfully filtering liquid droplets and gas bubbles as well as solidparticles.

In addition, the present invention is intended to provide a system forharvesting particles and regenerating the filtering medium byalternately activating and deactivating an acoustic field imposed on aporous filtering medium.

Additional advantages of the present invention will be set forth in partin the description that follows, and in part will be obvious from thatdescription or can be learned by practice of the invention. Theadvantages of the invention can be realized and obtained by the methodand apparatus particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

To overcome the problems of the prior art filtration processes, and inaccordance with the purpose of the invention, as embodied and broadlydescribed herein, the method of this invention for separating particlesfrom a fluid comprises flowing a fluid containing the particles througha porous medium while imposing an acoustic field on the porous medium.Preferably the porous medium is disposed in a chamber, and the acousticfield has a frequency resonant to the chamber filled with the fluid.

In a more specific embodiment, the method of this invention forseparating particles from a fluid comprises the steps of providing aporous medium within a chamber, the porous medium having a predeterminedaverage pore size, supplying the chamber with a fluid containing theparticles and causing the fluid to flow through the porous medium andout of the chamber; and imposing an acoustic field on the porous medium,the acoustic field causing the porous medium to trap particles having anominal diameter substantially less than the predetermined average poresize of the porous medium.

The method of this invention also can be used to “harvest” particlesfrom a fluid by flowing a fluid containing the particles through aporous medium while imposing an acoustic field on the porous medium, theacoustic field causing the porous medium to trap particles, removing theacoustic field from the porous medium, the removal of the acoustic fieldpermitting particles trapped in the porous medium to pass through theporous medium with the flowing fluid, and collecting the particlespassing through the porous medium after the acoustic field is removed

In accordance with another aspect of the invention, the apparatus of theinvention is for separating particles from a fluid and comprises achamber; a porous medium disposed within the chamber; means for flowinga fluid containing particles through the chamber and the porous mediumand means for imposing on the porous medium an acoustic field causingthe porous medium to trap particles. The porous medium preferably iseither a mesh or foam filter or a plurality of contacting solids. Theimposing means preferably generates an acoustic field having a frequencyresonant to the chamber filled with the fluid but without the porousmedium.

The accompanying drawings, which are incorporated in and whichconstitute a part of this specification, illustrate at least oneembodiment of the invention and, together with the description, explainthe principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of the apparatus of the invention asused to perform the method of the invention;

FIGS. 2(a) and 2(b) are schematic diagrams showing the extent ofparticle filtration in the apparatus of the invention before and duringapplication of the acoustic field on the porous filtering medium;

FIG. 3 is a schematic diagram of a particle-harvesting circuit employingthe method of the present invention;

FIGS. 4(a), 4(b), and 4(c) are schematic diagrams illustrating three oftheoretical mechanisms by which the method of the present inventiontraps particles having diameters substantially smaller than the averagepore size of the porous filtering medium;

FIG. 5(a) is a (graph of particle concentration versus time downstreamof the porous filtering medium during one illustrative example of themethod of the present invention;

FIG. 5(b) is a graph of particle mass retained in the porous filteringmedium as a function of time for the same example of the method of thepresent invention described in FIG. 4(a); and

FIGS. 6-17 are graphs of retained particle mass versus time foradditional illustrative examples of the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference now will be made in detail to the presently preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

One embodiment of the apparatus of the present invention is shown inFIG. 1. The filtering apparatus, designated generally by referencenumeral 10, is intended to separate from a host fluid stream particleshaving an average diameter of about 20 μm. The apparatus of theinvention includes chamber 12, which preferably is rectilinear with sidewalls 14 (only two of which are shown), bottom wall 16, and top wall 18.Disposed within chamber 12 is a porous medium 20 having a predeterminedaverage pore size. Porous medium 20 preferably comprises a conventionalaluminum mesh or polyester foam filter or unconsolidatedmillimeter-scale spherical glass beads.

Chamber 12 also is provided with means for flowing a fluid containingparticles through the chamber and the porous medium. As embodied hereinand as shown in FIG. 1, the fluid-flowing means includes fluid inlet 22and fluid outlet 24. As will be appreciated by one skilled in the art,the fluid-flowing means also would include a feed pump or gravity-feedarrangement (not shown) and a conduit (not shown) connecting the feedpump to inlet 22. Preferably, interposed between fluid inlet 22 and theinterior of chamber 12 is a porous mesh support 26, which aids indistributing the fluid and entrained particles throughout porous medium20.

In accordance with the invention, filtering apparatus 10 also includesmeans for imposing on the porous medium an acoustic field causing theporous medium to trap particles from the flowing fluid. As embodiedherein and as shown in FIG. 1, the field-imposing means of thisinvention includes a transducer 30 mounted on the interior face of oneside wall 14 and an acoustic reflector 32 mounted on the interior faceof the opposite side wall 14. Transducer 30 and reflector 32 can beidentical transducers, with only one being energized to impose anacoustic field across porous medium 20. Transducer 30 preferably isdriven at a frequency resonant to chamber 12 when the chamber is filledwith fluid, and most preferably without the presence of porous medium20.

We have found that, when such a resonant acoustic field is imposed on aporous filtering medium, the porous medium will trap particles havingdiameters substantially smaller than the average pore size of the porousmedium. Particles having nominal diameters up to three orders ofmagnitude smaller than the pore size are capable of being trapped whilethe resonant acoustic field is imposed.

FIGS. 2(a) and 2(b) are schematic illustrations showing the effect ofthe acoustically enhanced filtering method of the present invention. InFIG. 2(a), the acoustic field is not activated, and the particlessuspended in the flowing fluid pass readily through porous medium 20,which has an average pore size substantially larger than the diametersof the particles. When the resonant acoustic field is activated,however, a substantial proportion of the fine particles become trappedwithin the porous medium, as shown in FIG. 2(b). Subsequent removal ofthe acoustic field not only halts the trapping of particles fed to theporous medium, it allows the fluid stream to flush out of the porousmedium those particles that previously had been trapped by theinteraction of the porous medium and the acoustic field, therebyregenerating the porous medium.

A significant advantage of the present invention is that trapping ofsmall particles is accomplished without an accompanying drop in pressureacross the porous medium. A major disadvantage of conventionalfiltration systems, which rely on small pore sizes to trapsmall-diameter particles, is the substantial drop in pressure across theporous medium as the trapped particles clog the medium. The trappingmechanism of the present invention operates without such clogging.

As will be shown below, the trapping efficiency of the acousticallyactivated porous medium increases dramatically immediately after theacoustic field is imposed on the chamber, levels off for a period, andthen drops steadily until the porous medium is effectively saturated andunable to trap any additional particles. By selectively activating andde-activating the acoustic field, collection of particles can befacilitated. For example, the exit stream can be switched to acollection circuit during deactivation to harvest the formerlyacoustically trapped particles.

One such collection circuit is shown in FIG. 3, in which fluid outlet 24is split into two branches, one connected to solids recovery conduit 40and one to clarified liquid conduit 42. The connections to conduits 40and 42 are provided by valves 44 and 46, respectively. A sensor 48connected to controller 50 detects the particle concentration in thechamber effluent flowing through outlet 24. Preferably, sensor 48incorporates a photodetector that measures the amount of light that canpass through the effluent.

During, activation of the acoustic field, particles are trapped in theporous medium contained in the chamber, and controller 50 opens valve 46to permit flow of the clarified effluent from chamber 12 through conduit42. After the trapping efficiency of chamber 12 peaks, the effluentleaving the chamber contains an increasing concentration of particles.When sensor 48 detects that the particle concentration in outlet 24 hasreached a predetermined minimum value corresponding to an optimumtrapping condition, the acoustic field is deactivated, and the fluidstream flushes previously trapped particles out of chamber 12. Inaddition, controller 50 closes valve 46 and opens valve 44 to connectoutlet 24 to solids recovery conduit 42.

Numerous variations of the circuit shown in FIG. 3 can be used inpracticing the method of this invention. For example, the deactivationof the acoustic field can be accomplished directly by controller 50 orvia another controller that receives a signal from sensor 48 that thethreshold particle concentration in the effluent has been reached.Moreover, instead of splitting outlet 24 and employing two valves, asingle valve can be used to connect outlet 24 to either solids recoveryconduit 44 or clarified liquid conduit 46.

Alternatively, trapped particles can be harvested by removing the porousmedium 20 from the chamber 12 while the acoustic field is active, theparticles trapped in the porous medium would accompany the medium out ofthe chamber. One possible way of accomplishing medium removal would beto provide the chamber with a hinged wall that can be opened to removethe medium. If unconsolidated glass beads or other solids are used forthe medium, the medium and trapped particles can be readily removed fromthe chamber by gravity if the hinged wall is disposed beneath themedium.

The exact mechanism of particle retention in the porous medium by themethod of this invention is still under investigation. Although thepropagation of acoustic waves in a fluid-saturated porous medium hasbeen studied extensively, to our knowledge there has been no analysis ofthe acoustic force experienced by a small particle within the porousmedium.

We do know that, when an acoustic chamber is excited at a frequencycorresponding to resonance for the chamber, planar ultrasonic waves arepropagated within the porous medium through coupling by the liquidportion of the fluid suspension. As depicted in FIG. 2(b), when a fluidcontaining particles in suspension passes through the chamber, theparticles are retained within the porous medium due to the acousticradiation forces that act. For aqueous suspensions, the acousticimpedance of the liquid (1.5×10⁶ kg/m²s) is significantly different fromtwo of the types of porous media used in the studies detailed below(15.0×10⁶ kg/m²s for glass beads, 17.4×10⁶ kg/m²s for aluminum mesh) andonly one half that of the third type (3.0×10⁶ kg/m²s for polyesterfoam). Therefore, we expect the acoustic field to have a complexstructure within the chamber due to scattering from the porous media.

We perceive three possible effects that might give rise to theparticle-trapping phenomena we have observed within porous media in aresonant acoustic field. The first relies on primary acoustic forces,the others on secondary acoustic forces.

If the porous medium does not strongly interfere with the propagation ofthe primary sound field, it is reasonable to expect that particletrapping will occur via the primary acoustic force mechanism. In aone-dimensional resonant ultrasonic field, particulate solids (with sizemuch smaller than the acoustic wavelength) suspended in a liquid areknown to experience a time-averaged primary acoustic force. Forparticles with positive acoustic contrast (the case for most solidssuspended in water), this primary force causes the particles to collectat pressure nodes of the field. The role of the porous medium thus mightbe simply to prevent the particles from being entrained by the fluidflow. FIG. 4(a) depicts individual particles moved into positionscorresponding to the pressure nodes by the primary acoustic force. Asthe flow field tries to entrain the particles, their motion (along theflow direction) is blocked by the solid material within the porousmedium. Any motion of the particles around the obstruction is resistedby the primiary acoustic force, which acts to return them to one of thenodal positions where they again are blocked by the solid material ofthe porous medium.

Suspended particles subjected to a resonant acoustic field alsoexperience secondary acoustic forces, which arise due to interactionswith the acoustic field scattered by the particles. Secondary forcestend to be attractive between similar particles and also can causeparticles to be attracted toward the solids comprising the porousmedium. Thus, as shown in FIG. 4(b), secondary forces might formclusters of particles within the porous medium that are larger than thepore size and thus are trapped within the porous medium. Alternatively,as shown in FIG. 4(c), some particles might be so strongly anchored ontothe internal surfaces of the porous medium by secondary forces that thefluid flow is not be able to displace them. The secondary forces in turnmight cause additional particles to attach onto the anchored particles.

Whatever the mechanism underlying the phenomenon, the efficacy of thepresent invention is demonstrated by the following examples, all ofwhich utilized a rectilinear acoustic chamber as shown schematically inFIG. 1. The acoustic chamber was fabricated from Plexiglas and includedtwo rectangular lead zirconate titanate transducers (APC Model 880)measuring 76.2 mm×38.1 mm×6.25 mm, with a fundamental resonancefrequency of 320 kHz. The transducers were mounted on opposed walls ofthe chamber so that they were parallel to each other at a 5.8 mm spacing(except where otherwise noted).

Three types of porous media were used to fill the space between thetransducer and reflector. For experiments with unconsolidated media, 3mm diameter solid glass beads (Fisher Scientific, Pittsburgh, Pa.) wereused. When randomly packed within the acoustic chamber, the average sizeof the pores between the glass beads is approximately 300 μm. Otherexperiments were done with aluminum mesh (ERG Material and AerospaceCorp., Oakland, Calif.) and polyester foam (Stephenson & Lawyer Inc.,Grand Rapids, Mi.) having 10, 20, or 40 pores per inch (ppi). Theaverage pore sizes for the mesh and foam were 2000 μm for the 10 ppimedia, 1000 μm for the 20 ppi media, and 450 μm for the 40 ppi media.For all examples described (except otherwise noted), the dimensions ofthe space filled by the porous medium measured 70 mm high, 35 mm wide,and 5.8 mm thick in the direction of propagation of acoustic field. Inall cases the porous medium was supported on a 17.5 mm high metallicmesh positioned below the acoustically active volume to help thedistribution of the fluid flow within the chamber.

The acoustic field was produced by energizing one of the transducers ata frequency of 1.103 MHz using a signal generated by a KROHN-HITE 2100Asignal generator and amplified by an ENI 240L power amplifier. Thesecond transducer served as a reflector. The frequency chosencorresponds to one of the resonance frequencies of the acoustic chamberwhen filled with fluid but in the absence of the porous medium.Electrical power consumption was measured by a Bird Wattmeter Model4410A.

In determining the resonant frequency of the chamber we first assembledthe chamber and, without adding the porous medium, fed the chamber witha fluid containing particles in suspension. The powered transducer thenwas energized, and the frequency of the signal generator was adjusteduntil the particles within the suspension showed the formation of bandswithin the chamber at the nodal planes of the acoustic field. Thisphenomenon signaled the resonance condition. We then reassembled thechamber with the porous medium inside and energized the transducer atthe same frequency that produced the resonance condition in the absenceof the porous medium.

These experiments were performed using aqueous suspensions of 325-meshpolystyrene divinyl benzene particles (average particle diameter about20 μm). These particles were suspended in deionized water using TritonX-100 as a surfactant. A peristaltic pump was used to deliver thesuspension to the acoustic chamber. A set of three three-way valvesincorporated into the flow circuit allowed the feed to the chamber to besmoothly switched to clear water for some of the experiments describedbelow.

Particle concentrations in the suspensions were determined by measuringthe percentage fight transmittance at 360 nm in a BAUSCH & LOMBSpectronic 20 photometer. The transmittance values were converted toweight percentage solids using a calibration curve determined frommeasurements on standard suspensions.

Procedures

Feed suspensions of known concentrations of polystyrene particles inwater were passed through the chamber at fixed flow rates. Thetransducer was activated at a preselected power level to produce theresonant acoustic field. Because the densities of polystyrene particles(1.05 g/cm³) and water (1.00 g/cm³) are so close, the effects of gravitywere assumed to be insignificant.

Typically, the experiments were conducted by first establishing a flowof the suspension through the chamber while the acoustic field wasdeactivated. Following activation of the acoustic field, the particleconcentration in the exit stream at the top of the chamber was measuredat regular intervals of 30-60 seconds. Small samples of effluent(approximately 5 cm³) were collected and immediately analyzed using thephotometer to determine the particle concentration exiting the chamberThe elapsed time for each experiment was measured from the instance ofactivating the acoustic field.

In the experiments designed to quantify the effect of processingvariables for the glass-bead and aluminum-mesh media, a sequence ofthree segments was performed to test different performance features ofthe method.

Segment I: During the first segment, the suspension of a fixed particleconcentration was continuously fed into the chamber and an ultrasonicfield of constant frequency and intensity (maintained by constantvoltage supplied to the transducer) was maintained. Within less than aminute, the particle concentration in the exit stream typically droppedas particles were retained within the porous medium. Eventually, theporous medium became saturated with particles and their concentration inthe exit stream rose.

Segment II: After a preset duration of time, the feed to the chamber wasswitched to clear water while the acoustic field was maintained. Theflow rate of water was equal to that of the suspension feed rate inSegment I. The purpose of Segment II was to determine what fraction ofparticles, previously retained in the porous medium, could be removed byflushing with clear water while the acoustic field was energized. If theparticles were loosely trapped within the porous medium, the water flushcould be expected to remove them. Typically, a small fraction of theretained solids were found to be flushed from the chamber by this waterflushing. Segment II was continued until the concentration in the exitstream dropped to a low value (typically 0.05 wt %).

Segment III: During this final segment, the acoustic field in thechamber was deactivated while clear water continued to flow through atthe same flow rate as in Segment II. The particles still trapped insidethe porous medium due to acoustic forces would then be released. Thepurpose of this segment was to determine the amount of particlesretained due to the acoustic effects. Also, it provided some indicationof the length of time required to regenerate the porous medium.Typically, following the start of Segment III, the particleconcentration in the exit stream is quite high, but then it diminishedwithin a few minutes.

Qualitative Results

In all of the experiments performed, the application of the resonantacoustic field to the porous medium resulted in the retention ofparticles within the chamber. Within a few seconds of activating theacoustic field, the solids concentration in the effluent stream wasvisibly diminished in comparison to the feed suspension. In all casesstudied the porous medium was found to entrap the particles (averagediameter 20 μm), even though the pore size was much larger (up to 2000μm for the 10 ppi aluminum mesh). No collection was observed if theacoustic chamber was driven at a nonresonant frequency, or if theacoustic field was not energized.

It also became apparent that there was a maximum level of solids thatcould be retained by the porous media. Visual observation indicatedthat, after a time, the concentration of solids in the exiting streamincreases with continued use, indicating that the solids loading withinthe porous medium was reaching a saturation condition It was also clearthat the solids could readily be flushed from the porous medium when thesound field was deactivated.

In order to analyze the filtration performance of the method andapparatus of the present invention a series of quantitative experimentswere undertaken. These experiments focused on the saturation andflushing phenomena, as well as the relationships between filtrationefficiency and operating conditions.

EXAMPLE 1

FIG. 5(a) is a graph showing the variation of particle concentration inthe exit stream as a function of time for an experiment involving 0.3 wt% suspension fed at 35 cm³/min to the chamber with the transduceroperated with 20 W of power. This experiment used the unconsolidatedglass beads as the porous medium. In Segment I, the effluentconcentration reached a minimum within 1 minute of acousticallyenergizing the flow field. Thereafter, as the porous medium becamesaturated, the effluent concentration rose, reaching the originalparticle concentration after about 9 minutes, indicating the state offull saturation. When clear water was fed in Segment II, theconcentration of particles in the effluent dropped over the course of 3to 4 minutes, but the concentration did not drop to zero. This indicateda continuing leakage of solids from the porous media due to hydrodynamicentrainment. Upon deactivating the field in Segment III, a rapid releaseof the particles from the chamber was seen. Within a matter of 2 min.the chamber was essentially devoid of particles.

Corresponding to the results shown in FIG. 5(a), FIG. 5(b) shows theamount of particle mass retained inside the porous medium relative tothe total mass of particles fed to the chamber as a function of time.This cumulative retention percentage, η_(ret), can be taken as anoverall measure of filtration efficiency of the separation method.Values of η_(ret) were calculated from the concentration data by theequation${\eta_{ret} = {\frac{\int{\left( {C_{out} - C_{in}} \right) \times Q{t}}}{\int{C_{in} \times Q{t}}} \times 100}},$

where

C_(out)=particle concentration in the exit stream;

C_(in)=particle concentration in the feed stream; and

Q=suspension flow rate.

FIG. 5(b) indicates that, during the first 2 minutes, the retentionpercentage increases with time, reaching a maximum value ofapproximately 60%, but drops thereafter. After this peak, the filtrationefficiency decreases continuously. By the end of Segment I (9.25 min),approximately 35% of the total mass fed to the chamber is retained.During Segment II, the percentage retention continues to drop, with only12% of the fed particles being retained even at the end of Segment II.This does indicate, however, that some particle trapping is due to theeffects of the acoustic field. As the acoustic field is turned off atthe beginning of Segment III, most of the remaining trapped particlesexit the porous medium in a very short time period (indicated by sharpdrop in retention percentage). The very small negative value at the endof Segment III is an indication that the material balance on theparticles closes quite well.

Experiments were repeated for a range of suspension flow rates, feedconcentration, and levels of electrical power supplied to thepiezoelectric transducer.

EXAMPLE 2

The effect of suspension flow rate is shown in FIG. 6 for a 0.3 wt %feed concentration through the glass-bead medium and with the transduceroperated at 20 W. As the suspension flow rate was increased from 30 to45 cm³/min the residence time of the suspension in the porous mediumdecreased, and therefore one expected to see a decrease in filtrationefficiency. FIG. 6 shows that the filtration efficiency was unaffectedby the flow rate during initial period, but the time at which thepercentage retention reached its peak value and the value of maximumretention decreased with increasing flow rate. The percentage retentionfor higher flow rates were consistently below those for lower flow ratefor the whole duration of Segments I and II.

EXAMPLE 3

The effect of feed concentration is shown in FIG. 7. For this set ofexperiments, also using the glass-bead porous medium, the flow rate wasfixed at 35 cm³/min, electrical power was 20 W, and feed concentrationsof 0.3, 0.4 and 0.5 wt % were used. In Segment I, there was very littlevariation in percentage retention for the various feed concentrations.Small differences are seen in Segment II, and these differences canprobably be attributed to experimental error. These observationsindicate that the filtration is not limited by filling of the availablepore volume, at least for the range of feed concentrations tested.

EXAMPLE 4

The acoustic field intensity in the chamber is directly related to theelectrical power consumed in the piezoelectric transducer. The exactrelationship between electrical power consumption and the acoustic fieldintensity, however, is complex (and usually unknown because of unknownlosses in acoustic chamber). Thus, we use the electrical powerconsumption as a measure of the acoustic intensity. FIG. 8 shows therelationship between electrical power consumption and percentageretention for a fixed feed concentration of 0.2 wt % and a flow rate of40 cm³/min through the glass-bead porous medium. As can be expected,higher filtration efficiency was observed at higher power levels.Similar experiments were carried out for feed concentrations of 0.4 wt%. In this case, there was an improvement in the filtration efficiencywhen the power was increased from 20 to 310 W.

EXAMPLE 5

FIG. 9 shows the effect of suspension flow rate on the cumulativefiltration efficiency for feeds of 0.3 wt % with the chamber filled withthe 10 ppi aluminum-mesh porous medium and the transducer driven with 20W of power. The percentage retention in both Segments I and II, as wellas the time at which percentage retention starts to decrease from itsmaximum, was significantly higher with the aluminum mesh than with theglass beads. This improvement in filtration efficiency compared to thecase of the unconsolidated spheres might be attributable to the higherporosity of the aluminum mesh (92-94% compared to 35% for the glass beadbed). This leads to a slower interstitial velocity inside the porousmedium and hence reduced tendency of the carrier fluid to entrainparticles.

EXAMPLE 6

FIG. 10 shows the influence of feed concentration on filtrationefficiency for a feed flow rate of 40 cm³/min using a chamber equippedwith the 10 ppi aluminum-mesh porous medium and powered at 20 W. DuringSegment I, there was no significant effect of feed concentration onfiltration efficiency, which is similar to the results seen for the caseof the unconsolidated-sphere medium. In segment II, however, thepercentage retention shows a strong effect offered concentration in therange from 0.3. to 0.4 wt %. This indicates that the particles retainedfrom the feed with higher concentration were in a configuration moresusceptible to entrainment than those particles entrapped from a lowerconcentration feed. Correspondingly, the configuration of the entrappedparticles within the porous medium might be sensitive to the kineticswith which the particles are collected within the porous medium.

EXAMPLE 7

FIG. 11 shows the effect of acoustic field intensity in the acousticchamber, again expressed in terms of electrical power supplied to thepiezoelectric transducer, on the filtration performance. In this case,the chamber was equipped with 10 ppi aluminum mesh, and a 0.2 wt %suspension was fed at 40 cm³/min, As expected for Segment I, thefiltration efficiency increased with the power supplied. Segment IIshows a more extensive removal of particles collected at high power,once again indicating that the configuration of these particles withinthe pores makes them more susceptible to entrainment and flushing. Aswas the case with the unconsolidated medium, an increase in the feedconcentration leads to a reduced sensitivity to power level.

EXAMPLE 8

The pore size in the porous medium directly affects both the specificvelocity and the internal surface area available for particleattachment. FIG. 12 shows how filtration performance is affected by poresize for a chamber operated at a fixed flow rate of 40 cm³/min of 0.3 wt% feed with 20 W of power, using aluminum mesh filters with pore sizesof 10 ppi, 20 ppi. and 40 ppi. Similar results were obtained withpolyester foam media having these pore sizes. In general, filtrationperformance improves with a decrease in pore size. Since smaller porescorrespond to more internal surface area, this result suggests that, asdepicted in FIG. 3(b), secondary acoustic forces may be responsible forthe particle collection.

We also have shown in FIG. 12 the percent retention in the chamberobtained when there is no porous medium present between the transducerand reflector. In the absence of a porous medium, the small percentageretention observed may be attributed to radiation induced agglomerationand sedimentation of agglomerated particles due to gravity. It is clearthat the presence of the porous mesh has a pronounced effect on thefiltration performance.

EXAMPLE 9

FIG. 13 shows the effect of suspension flow rate on the cumulativefiltration efficiency for feeds of 0.5 wt % with the chamber filled withthe 10 ppi polyester foam porous medium and the transducer driven with20 W of power. In this example, only the results during the Segment Iphase are shown. As with the glass-bead medium and the aluminum mesh,increasing the flow rate shortened the time to maximum filtrationefficiency. The percentage retention at a flow rate of 30 cm³/min wassimilar that of the 10 ppi aluminum mesh at the same flow rate (see FIG.9). At higher flow rates, the polyester foam exhibited increasedretention when compared to the aluminum mesh.

EXAMPLE 10

FIG. 14 shows the influence of feed concentration on filtrationefficiency for a feed flow rate of 30 cm³/min using a chamber equippedwith the 10 ppi polyester foam porous medium and powered at 20 W. Again,only the results during Segment I are shown. There was no significanteffect on filtration efficiency due to feed concentration during thefirst 4 minutes. After that, however, the efficiency dropped rapidly forthe sample using a feed concentration of approximately 1.0 wt % butcontinued to rise for the sample using a feed concentration ofapproximately 0.5 wt %. Filtration efficiency for the test using thelower feed concentration reached a maximum at about 10 minutes.

EXAMPLE 11

FIG. 15 is another graph showing the effect of acoustic field intensityin the acoustic chamber, again expressed in terms of electrical powersupplied to the piezoelectric transducer, on the filtration performanceduring Segment I. In this case, the chamber was equipped with 10 ppipolyester foam, and a 0.5 wt % suspension was fed at 80 cm³/min. Asexpected, the filtration efficiency increased with the power supplied,and the high feed concentration value shortened the time at whichmaximum filtration efficiency was reached.

EXAMPLE 12

Tests were done to evaluate the effect of scaling up the method of theinvention to a larger chamber. FIG. 16 is a graph showing, the effect ofvarying size of the chamber and the resulting change in the distance orgap between the powered transducer and the reflector. First, thestandard 5.8 mm deep chamber was used to establish a baseline. Thechamber was filled with 10 ppi polyester foam, a 0.5 wt % suspension wasfed at 40 cm³/min, and the transducer was operated at 20 W. Next, a 12.5mnm deep chamber was fabricated and filled with 10 ppi polyester foam,and the feed flow was increased to 80 cm³/min to keep the residence timeof the suspension feed approximately the same. When this larger chamberwas acoustically activated at 20 W of power, the maximum filtrationefficiency dropped by about 15%. We then doubled the power fed to thetransducer energizing the larger chamber (to 40 W), which resulted inrestoring the filtration efficiency to a level substantially equivalentto that when the transducer was operated at 20 W in the 5.8 mm deepchamber.

EXAMPLE 13

FIG. 17 is a graph of cumulative mass retention (in grams) as a functionof time for two different resonant frequencies. In this case, thechamber was equipped with 10 ppi polyester foam, a 0.5 wt % suspensionwas fed at 30 cm³/min, and the transducer was operated at 20 W of power.For the two different frequencies, the mass retention was substantiallyequal for the first 3 minutes of particle capture. After 3 minutes, theporous medium excited at 1.103 MHz exhibited a slightly higher retentionefficiency than that excited at 1.852 MHz.

It is clear that application of resonant acoustic fields to porous mediacan result in their functioning, as filters for particles much smallerthan the pore size of the medium. Reasonable filtration efficiencieswere achieved in the laboratory tests. Large fractions of particles weretrapped from flowing, suspension (Segment I in the experiments.) Inpractice, the loss of particles as the feed would be switched to clearwater (Segment II) or the field deactivated (Segment III) indicates theease with which the porous medium can be regenerated. The filtrationefficiency was observed to follow the expected trends with respect tovariation in suspension flow rate and acoustic field intensity.Filtration efficiency was found to be almost independent ofconcentration of particles in the feed suspension in the range of 0.3 to0.5 wt %. The higher efficiencies observed for the aluminum mesh andpolyester foam media relative to the glass-bead medium could beattributed to their higher porosities and the corresponding smallerinterstitial velocities.

The residence time of suspensions in the active acoustic field (thevolume of the active acoustic chamber divided by the volumetric flowrate of the fluid through the chamber) varied from 0.32 to 0.47 min forflow rates between 30 and 45 cm³/min. In case of the unconsolidatedmedium, the filtration efficiency reached its maximum after 1.5 to 4 minof operation, after which it started to decrease. The correspondingdurations were 5 to 8 min for the 10 ppi aluminum mesh and 6.5 to 10 minfor the polyester foam (omitting the samples run at extreme conditionsrelative to the aluminum mesh). Thus the filtration process could becontinued for about 5 residence times in the unconsolidated media, forabout 16-17 residence times for the aluminum mesh, and for about 20-21residence times for the polyester foam before losing filtrationefficiency.

Separation efficiencies in excess of 90-95% have been reported inearlier resonant ultrasonic field-based filtration and fractionation.The continuous nature of operation and high efficiency makes thosemethods very attractive. They have operational difficulties, however,when large quantities of suspensions need to be treated. The filtrationmethod of the present invention is much more amenable to large scaleoperations. Additionally, there is no need to use closely spacedphysical barriers, and one should be able to achieve reasonableefficiency even in larger volumes of porous medium as long as ultrasonicfields of appropriate intensities are employed. Higher overallfiltration efficiency may be achieved by using multiple states.

It will be apparent to those skilled in the art that other modificationsand variations can be made in the method of and apparatus of theinvention without departing from the scope of the invention. Forexample, mesh and foam filters formed from materials other than aluminumor polyester can be used as the porous medium. Similarly, unconsolidatedporous media can be produced using solids other than glass spheres, suchas Raschig rings, Berl Saddles, and other shapes. In addition, althoughthe examples set forth above exhibit acoustically enhanced filtrationwhen the frequency of the acoustic field was resonant to the chamberfilled with liquid but without the porous medium, it is to be expectedthat the method of the invention also will operate at frequenciesresonant to the liquid-filled chamber with the porous medium present.The invention in its broader aspects is, therefore, not limited to thespecific details and illustrated examples shown and described.Accordingly, it is intended that the present invention cover suchmodifications and variations provided that they fall within the scope ofthe appended claims and their equivalents.

We claim:
 1. A method for separating particles having a size in therange of 1-100 μm from a fluid, comprising the steps of: a. providing aporous medium within a chamber, the porous medium having an average poresize of about 450 μm or greater; b. supplying the chamber with a fluidcontaining the particles and causing the fluid to flow through theporous medium and out of the chamber; and c. imposing an acoustic fieldon the porous medium, the acoustic field causing the porous medium totrap particles having a nominal diameter of the range of 1-100 μm. 2.The method of claim 1, wherein the acoustic field has a frequencyresonant to the chamber when the chamber is filled with the fluid. 3.The method of claim 1, wherein the acoustic field has a frequencyresonant to the chamber when the chamber is filled with the fluid but inthe absence of the porous medium.
 4. A method for separating particlesfrom a fluid, comprising the steps of: a. providing a chamber having aninlet and outlet; b. providing a porous medium within the chamberbetween the inlet and the outlet, the porous medium having apredetermined average pore size; c. supplying a fluid containing theparticles to the inlet of the chamber and causing the fluid to flowthrough the porous medium and out of the outlet of the chamber; d. afiltering step of imposing an acoustic field on the porous medium, theacoustic field causing the porous medium to trap particles having anominal diameter substantially less than the predetermined average poresize of the porous medium; and e. a regenerating step of removing theacoustic field from the porous medium to permit particles trapped in theporous medium during the filtering step to pass through the porousmedium with the flowing fluid and be removed through the outlet of thechamber.
 5. The method of claim 4, wherein the acoustic field has afrequency resonant to the chamber when the chamber is filled with thefluid.
 6. The method of claim 4, wherein said filtering step isperformed until the amount of particles trapped in the porous mediumreaches a predetermined level and then said regenerating step isperformed.
 7. The method of claim 4, further including the step ofdetecting the level of particle concentration in the fluid flowing outof the chamber during the filtering step, the regenerating step beingcommenced when the detected particle concentration reaches apredetermined level.
 8. The method of claim 4, further including thesteps of: connecting the outlet of the chamber to a first fluid conduitduring the filtering step; and connecting the outlet of the chamber to asecond fluid conduit during the regenerating step.
 9. The method ofclaim 4, wherein the acoustic field has a frequency resonant to thechamber when the chamber is filled with the fluid but in the absence ofthe porous medium.
 10. A method for separating particles from a fluid,comprising the steps of: flowing a fluid containing the particlesthrough a porous medium while imposing an acoustic field on the porousmedium, the acoustic field causing the porous medium to trap particles;removing the acoustic field from the porous medium, the removal of theacoustic field permitting particles trapped in the porous medium to passthrough the porous medium with the flowing fluid; and collecting theparticles passing through the porous medium after the acoustic field isremoved.